Bring back initPolymer, allow boot.js only if using "polymer_experimental.html".

pkg/polymer/test/custom_event_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library polymer.test.web.custom_event_test;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'package:template_binding/template_binding.dart' show nodeBind;
@@ skipping 24 matching lines @@
 class TestCustomEvent extends PolymerElement {
   TestCustomEvent.created() : super.created();
 
   get fooBar => getShadowRoot('test-custom-event').querySelector('foo-bar');
 
   final events = [];
   fooHandler(e) => events.add(['foo', e]);
   barBazHandler(e) => events.add(['barbaz', e]);
 }
 
-@initMethod
-main() {
+main() => initPolymer().run(() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
 
   test('custom event', () {
     final testComp = querySelector('test-custom-event');
     final fooBar = testComp.fooBar;
 
     final binding = nodeBind(fooBar).bindings['on-barbaz'];
     expect(binding is Bindable, true,
         reason: 'on-barbaz event should be bound');
 
     expect(binding.value, null, reason: 'event bindings do not have value');
 
     fooBar.fireFoo(123);
     fooBar.fireBarBaz(42);
     fooBar.fireFoo(777);
 
     final events = testComp.events;
     expect(events.length, 3);
     expect(events.map((e) => e[0]), ['foo', 'barbaz', 'foo']);
     expect(events.map((e) => e[1].detail), [123, 42, 777]);
   });
-}
+});